A typical magnetic structure of a superconducting cyclotron, illustrated for example in documents U.S. Pat. No. 7,656,258 and WO2012/055890, comprises a cold mass structure including at least two superconducting magnetic coils, i.e. magnetic coils which comprise a material that is superconducting below a nominal temperature. A cryostat generally encloses this cold mass structure and forms a vacuum chamber for keeping the cold mass structure under vacuum. The cold mass structure is cooled with one or more dry cryocooler units below the nominal temperature at which the magnetic coils are superconducting. A disadvantage of using a cryostat which encloses only the coils is that a plurality of openings must be provided in the magnetic structure, be it in the upper and lower part of the magnetic yoke (as in U.S. Pat. No. 7,656,258), or in the surrounding return yoke (as in WO2012/055890), for allowing the passage of the cryocooler units to the cryostat. These openings are increasing the technical complexity of the installation as well as representing a disturbance of the magnetic circuit. Further technical complexities in these designs follow from the requirement of a coil support (referred to as a bobbin), for supporting the coils and a plurality of tie rods for maintaining the coils in place within the cryostat. As an alternative to dry magnets and dry cryocoolers, wet magnets may be used also.
Another approach is to enclose the totality of the magnetic structure into the interior of a cryostat, as shown in document US2012/0126726. In this cyclotron, the cold mass includes the coils as well as the magnetic yoke structures above and below the coils. The beam chamber in which the ions accelerate under the influence of an alternating voltage must however be isolated from this cold mass, thus requiring a super-insulating layer between the magnetic poles and said beam chamber. The disadvantage of such an isolation layer is that it increases the magnetic gap between the poles of the magnetic structure, which in turn requires a higher pole radius in order to take into account magnetic field losses. Another drawback of the latter approach is that the poles cannot be dismounted during the magnetic mapping phase without opening the cryostat.